Think plumber's P-trap. The one under your sink serves to keep sewer gases from backing up into your house. Liquid is held in place by gravity and provides a barrier vapors can't get through.

In refineries some form of liquid seal is typically found in vacuum system seals, flare drums, sulfur plant rundown legs, tank overflows etc. They're usually in place to prevent flammable or toxic vapors from draining out with liquids.